TAIL WIND LIMIT ON LANDING AND T/O

[bob3]

I was wondering that why is that almost all aircraft have 10 kts of tail wind limit and why is this tail limit there.
Is the limit only applicable when we are field lenght limited .Please confirm?

[AffirmBrest]

Most people (myself included) would want to use the other runway if the wind were greater than 10kts or so up the chuff.

Although there must be a few single-direction runways in the world, I can't see the authority requiring the manufacturer to spend ages certifying for a situation which is unlikely to occur in operation. Not for big aeroplanes anyway.

Or is there?

[Silky]

Some of the Canaries Islands are very affected by this limit, even the newer A5 powered A320 cannot make it non stop to the UK, but after saying all that I believe that Airbus are producing special performance charts to allow take off with up to a 15kt tail wind. :) :) :)

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Gravity always wins....

[John Farley]

Bob3

Short answer:

With a tailwind an aircraft will not flare so well, will be harder to keep straight during the initial part of the roll out and will take a lot further to stop

Long answer:

The effects of landing in a tailwind all stem from the extra speed over the ground.

BEFORE touchdown we need to remember the increased kinetic energy of the aircraft that will be present. The increased KE will, thanks to Sir Isaac Newton, mean that the aircraft is (marginally) less manoeuvrable for any given input. For pilots used to the IAS/TAS issues where at high altitude aerodromes the same effect is happening (flying usual IAS but higher TAS therefore more inertia) it will represent little more than a need to recognise the aircraft will be a tad more sluggish – especially in the flare. HOWEVER, inertia effects are of course V squared jobs, so with a Vref of say 140 with a headwind of 25 kts the inertia effects are related to 140 minus 25 = 115 which when squared is 13225. With the same 25 kt wind up your chuff the inertia effects are related to 140 plus 25 = 165. When this is squared we get 27225 i.e over a 100% increase when compared to the into wind case. Such an increase as that will certainly come through in the handling as a marked lack of response compared to what one is used to.

AFTER landing the higher ground speeds will (thanks to the same inertia business) mean the aircraft will be more reluctant to steer as accurately as you are used to at lower speeds. Again it will appear sluggish (or a bit as if the R/W is slippery).

I’m sure we don’t need to remind ourselves that all the extra KE will thrash the tyres and brakes when it comes to stopping, unless you really do have a LOT of length to spare.

Finally, for completeness there are tyre limiting speeds. But these are more likely to get into the high AUW high altitude take off case than during a fast touchdown.

Bottom line, there are some potential handling aspects to downwind landings that may need to be thought about as well as the obvious stopping issues.

JF




[This message has been edited by John Farley (edited 07 November 2000).]

[mutt]

Boeing offers an increased tailwind limit of up to 15 knots for a fee.

Mutt

[quid]

In the US, "Industry standard Ops Specs" put a 10 kt tailwind limit on operations.

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[bob3]

That does explain ( to an extent) why the limit is there for approach, but then why is this limit present for T/O, specially if we are not R/W lenght limited.

[Capt Pit Bull]

Performance isn't my thing, but for a wild stab in the dark:

Wind gradients are going to all be performance decreasing rather than performance increasing?

CPB

[AffirmBrest]

True; also as JF mentioned, tyre speed limut would play an increasing role with a tailwind, on TO as well as LDG.

Limits on an aircraft performance are often there just because they represent the extent to which the aircraft was test-flown during manufacture - usually to conform with authority minimums. Just because an aeroplane has a 10kt tailwind limit doesn't NECESSARILY mean it WOULDN'T T-O with a greater t/w, just that that is the extent to which it has been tested.

As I suggested before, why test it with a stronger t/w when 99 times out of 100 you could just use the other runway? Thus there is no reqt to demonstrate higher t/w T-Os.

For the record - I have no ambition to be a TP...I stay WELL within the limits :)

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...proceeding below Decision Height WITH CAUTION...

[bob3]

Well my qusetion was different folks .According to A.F. it seem that it is a certification problem and J.F. think it is cause of the flare problem and also due to the extra stopping distance (which i beleive is when we are R/W limited ),however I just dont understand, what is the effect on the performance of an aircraft(say b-737 or any airliner) if the pilot does T/O orland, at the max tail wind limit and also how come that there is same tail wind limit for 747 and 737 and also for small turboprop.

[mutt]

A: Performance wise, a tailwind will have the same effect as using a shorter runway or having closer obstacles. There is nothing stopping a pilot taking off with a 10 knot (or 15 knot) tailwind provided that he has accounted for it in his takeoff calculations. There will be a weight decrement.

B: Have a look at FAR25, you will probably find that all aircraft must be certified to a minimum of 10 knots tailwind. Manufacturers do not want to spent more money than required to certify an aircraft, they will therefore only flight test to the required limits. Our newer aircraft have a 15 knot tailwind limit, it was offered by Boeing and MDD as a cost item.

Mutt

[oak]

The Dash 8 100/200/300 has normally a standard TW limit of 10 kts, but with special supplement that was of course flight tested up to 20 kts may be used. Use of this supplement is limited to higher flapsettings because otherwise problems with brake energy (also for take-off -because you must be able to stop with full braking) and possibly controllability could happen. I don't think it would be practical to have even higher TW components. Already with 15 kts tailwind take-off, you need a strong copilot to push down on the (unpowered) elevator.Only recommended for "oneway airports".

[sets]

I think the tailwind takeoff limit is to prevent engine surges (inlet instability) on turbofans.

[alosaurus]

In terms of the dry take off case,increased groundspeed during an abort causes a huge increase in stopping distance.Viscous hydroplaning can occur with very little water on the runway and a 10 knot tailwind would,in this case, bring the onset of hydroplaning down to an IAS around,typically, 55 knots.On a slippery runway this would make most a/c field length limited for t/o.There was a useful thread on hydroplaning a couple of weeks ago.

[This message has been edited by alosaurus (edited 09 November 2000).]

[quid]

JF-

I'm glad to see you qualified the Kinetic energy arguments with the terms "marginally" and "just a tad", etc.

I think the following need to be recognized:

1. The a/c flies in the relative wind. Responses to control inputs will be no different until some "ground based" reference is used, i.e., ILS loc and g/s or runway. With a higher groundspeed, the inputs for corrections to the loc or g/s will "appear" to be sluggish due to the differing rates of corrections to the off-course indications.

2. Due to the increased groundspeed with a tailwind component, the rate of descent for a 3 degree glidepath will be greater, so more control input will be necessary to flare, if started at the same absolute altitude. It will seem "sluggish".

3. "Ground effect" will be more pronounced with a headwind than a tailwind. In the flare that would certainly be a factor.

4. After touchdown, groundspeed will be a big factor. After touchdown, the only airfoil still "flying" is the rudder. At any given groundspeed, the effect of the headwind or tailwind will be markedly different. The rate of a/c response to rudder inputs with a 120 kt groundspeed and a 20 kt headwind will be much greater than a 120 groundspeed and a 20 kt tailwind.

To keep comparing apples to apples, when at high density altitude airports, the IAS/TAS relationship will not change with variations in headwinds/tailwinds, but the above considerations will have a marked effect.

Agreed?

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[quid]

bob3-

My Ops Specs (and AFM) limit me to 10 kts tailwind component for both t/o and landing.

Is it "possible" to operate at higher values? Sure, given enough runway I can both takeoff and land. It just ain't legal.

I don't know "why" 10 kts was chosen. I guess it's just a reasonable round number. It's been around a long time. (Way before high bypass turbofans.)



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[manuel ortiz]

OUR 319'S A/I FCOM'S SHOW A MAX OF 15 KTS TAILWIND FOR LDG .

[John Farley]

Quid

I think I broadly agree with you! Please excuse the nitpicking that follows!

You say

1. “The a/c flies in the relative wind. Responses to control inputs will be no different until some "ground based" reference is used.”

I say

Yes but TAS is a ground based reference.

When I was a very youg lad I was taught to do loops at moderately low levels in a Vampire. I was then made to go up to 40,000ft and do the same. Ha!. When low level there was nothing wrong with pulling up at 230 kts IAS (giving say a TAS of 250 at a sensible height above the deck) HOWEVER, try that at 40k when the aircraft had a TAS twice its IAS and you soon learned about the effects of inertia on the tendency of an aircraft to go (relatively) straight on when you applied a given force (still 230kts worth in this case)

As you rightly say moving down an ILS at two different ground speeds gives two different rates of descent that have to be reduced in the flare.

You say

2. "Ground effect" will be more pronounced with a headwind than a tailwind. In the flare that would certainly be a factor.

I say

You may well be right for a largish - say commercial aircraft. But I just have a little niggle that the wind speed just above the runway will be less than at say 100 ft (windshear due to boundary layer effects if you like) this actually reverses its effect when landing with a tailwind i.e. you finish up with less of a tailwind right at the end, so IAS in the flare will decay slower than usual – which is a tad of good news in a tailwind landing. Mind you I would not like to measure the difference in a normal real turbulent atmos would you?

You say

3. After touchdown, groundspeed will be a big factor. After touchdown, the only airfoil still "flying" is the rudder. At any given groundspeed, the effect of the headwind or tailwind will be markedly different. The rate of a/c response to rudder inputs with a 120 kt groundspeed and a 20 kt headwind will be much greater than a 120 groundspeed and a 20 kt tailwind.
To keep comparing apples to apples, when at high density altitude airports, the IAS/TAS relationship will not change with variations in headwinds/tailwinds, but the above considerations will have a marked effect.

I say

Agreed

Good to talk to you.

JF

[bank angle]

Most manufacturers will certify aircraft up to 10 kts. tailwind or flaps up to 20 000 ft.or Take off thrust for up to 5 minutes, or 8200 ft. maximum field elevation, etc. to keep the cost of certification low, that doesn’t mean that the airplane can’t fly above this limits, as an example I can tell you that our 737s. are certified at 15 kts for take off and landing (sometimes landing with a tailwind is better than a low circle approach at night in rough terrain) or some airlines using high airport elevations can use their engines for 10 minutes at take off power in case of engine failure or take off and land at 13 000 ft. high fields.
This way the manufacturer keeps down the price of the aircraft for most airlines that do not need to operate under these conditions, and charges an extra fee to the airlines that need a special certification.
Certainly there is a limit, but not even the manufacturer may know it, maybe you can use flaps up to 26 000 or 28 000 ft, or 22 kts. tailwind, it is just to expensive to find out.
Ever wonder why the 737-400 or 500 can’t use flaps one for Take off like the 200 and 300, maybe it has to do only with certification cost and not with performance.
The more certification test they run the more expensive the aircraft they sell.

[quid]

JF,

I think the "loops" argument doesn't apply here. At different altitudes, air density and aerodynamic ceiling come into play. But, we may meet again on this subject on a future thread.

In any event, I appreciate your well reasoned response. Too often lately it seems that the mildest disagreements turn into childish name calling. I don't play that game. I enjoy technical debates with my fellow professionals.

Thank you.

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[Compliant One]

One factor frequently forotten is the fact that the winds are factored in performance calculations. Half the eadwind and twice the tailwind are used in the T/O case.
If you work his out then it's very penalising for the tailwind case ad you come to the end of any usable limits very quickly.

For reasons of operational flexibility, t's great for airports and airlines to be able to accept a tailwind. Too much, however, encourages people to think that it's OK.

Care in selction of runway is basic airmanship.

[Grandad Flyer]

Silky, OK, you've lost me. You said:

"Some of the Canaries Islands are very affected by this limit, even the newer A5 powered A320 cannot make it non stop to the UK"

What A5 engine? And which Canary Islands are you talking about? The only one I can think of where you might be performance limited is perhaps Lanzarote 03 with a strong tailwind, you would have a large weight decrement for this. But if that were the case you would use runway 21.
I have never known of an occasion where a tech stop was required for an Airbus from the Canaries to the UK.
I think all the Canary Island airports have a runway you can use from either end.
Please give us more information.